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Nanomaterials and surface chemistry


Plasmonic nanomaterials such as gold and silver, with their high extinction coefficient and their sensitivity to the refractive index of the surrounding medium, are useful labels for evanescent wave absorbance based sensors. Preparation, characterization, bio-conjugation and surface immobilization of these nanomaterials onto silica or plastic U-bent optical fiber probe surface is a challenge and several protocols have been utilized successfully by our group.

Nanomaterials

Materials having at least one of its dimension in nano regime i.e 1-100 nm (on average) are termed as nanomaterials. Based on dimensional confinement nanomaterials can be classified as (i) two dimensional (2D) nanomaterials where one of the dimensions is confined to nano regime eg: nanosheets and thin films, (ii) one dimensional (1D) nanomaterials, where two of its dimensions are confined to nano regime eg: nanorods, nanotubes, nanowires and (iii) zero-dimensional (0D) nanomaterials, where all the three dimensions are confined to nano regime eg: nanoparticles, quantum dots. Nanomaterials of different dimensions are synthesized/obtained through top-down approach (bulk to nano) or bottom-up approach (atoms to nano).

Nanomaterials exhibit dynamic physical properties in comparison to that of its bulk form due to a drastic increase in the surface area and confinement. Increase in the surface area leads to high surface energy and the particles become more reactive. Change in the aspect ratio (shape) of the particle significantly affects the confinement and thus the alters the quantized energy of electrons within the particle. The effect of confinement within the nano material could be clearly explored by considering particle in a 1D box, 3D box and in well.
Shape and size effects: The shape of the gold nanoparticles is a key parameter that determines the plasmonic absorption wavelength. Spherical particles are isotrophic and have a single absorption peak. Anisotrophic particles like rods or pyramids have a longitudinal and a transverse peak. There is a red shift in the absorption wavelength of nanoparticles, with increase in size.

Wet chemical synthesis Chemical route of nano particle synthesis is a bottom-up approach where the nano material is built from the atomic scale forming atomic clusters and then nano structures. Common routes of chemical synthesis include sol-gel, chemical reduction, template-based electrodeposition, and co-precipitation. Chemical reduction is widely exploited for preparation of gold and silver nano particles, where the metal ions are reduced to form clusters which on growth results in the formation of nanoparticles. The size of the particle is controlled by using surfactants which encapsulate the surface of the nano particle and inhibits further aggregation and growth. Gold nanospheres used in our group, is synthesized using citrate reduction method.

Physical vapor deposition Gold or silver thin films could be deposited by phyisical vapor deposition method. Sputtering of thin film in a low pressure system such as DC magnetron sputtering system gives rise to controlled deposition of thin films on target surface. Go to the publication
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Graphene oxide Research for increasing the surface area of the sensor by deposition of graphene was attempted.

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Surface chemistry

Silanization: These fiber probes were immersed in sulphochromic solution (100 ml conc. H2SO4 + 1 ml of 0.5 mg/ml K2Cr2O7) for 10 min, washed in DI water and dehydrated for 2 h at 115 ◦C to generate silanol sites). Fibers were silanized by dipping in 1% aminosilane in ethanol: acetic acid (10:4 v/v) mixture for 5 min and washed thrice in ethanol to generate amine functional groups. The pH of silane solution was maintained at 3.4 in order to minimize adsorption of protonated amine groups with silanol sites. These fiber probes were placed in a hot-air oven at 110 ◦C for 20 min for condensation. Gold nanoparticles could be immobilized on amine functionalized probes.

AminationThe surface of PMMA fiber probe was amine functionalized using Hexamethylenediamine as described by Fixe et al. Gold nanoparticles are immobilized onto the amine functionalized probe to generate an plasmonically active U-bent RI sensor.

Plasma functionalization PMMA surfaces could be made hydrophillic by plasma functionalization. Go to the publication
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Dendrimer functionalization Dendrimers could be used for functionalization of probes to increase the density of functional groups. Over the past few decades, dendrimers have been widely explored for surface modifications of various technologically important substrates including silicon, glass, mica, gold, quartz and silica. Dendrimer supramolecules have garnered much interest for designing sensor matrix because of the unique combination of their dendritic properties and capability to generate three-dimensional, highly dense functional sensor matrix which can help in amplification of the sensor signal. In addition, the ease in dendrimer synthesis chemistry and conjugation chemistry provide flexibility to choose dendrimer with homo or hetero multifunctional groups to generate the desired functionalities at sensor surface.

Bio-conjugation Antibodies were conjugated to gold nanoparticles (AuNP) by exploiting the affinity of amine groups to gold surface. The number of antibodies required to saturate the AuNP surface of varying sizes were theoretically calculated by taking total surface area of each AuNP and footprint area of the antibody. The free surface on the AuNP was blocked with SH-PEG. The concentration of antibody and SH-PEG for conjugation was resolved based the theoretical calculation.